Distributed optical sensors for acoustic and vibration monitoring

Abstract

An apparatus for sensing vibration in a borehole penetrating the earth includes an optical fiber having a plurality of pairs of reflectors configured to reflect light between each of the pairs of reflectors in the plurality, wherein each pair of reflectors is separated by a nominal distance L and an adjacent pair of reflectors in the plurality are separated by a nominal distance D, and an optical interrogator configured to sense a distance between each pair of reflectors in the plurality over time to sense the vibration. A laser emits semi-coherent light over a swept range of wavelengths to illuminate the optical fiber, wherein the semi-coherent light provides for sensing over distance L in the optical fiber between each pair of reflectors and for no sensing in the optical fiber over distance D between different pairs of reflectors. A photodetector senses light received from the optical fiber.

What is claimed is: 
     
       1. An apparatus for sensing vibration in a borehole penetrating the earth, the apparatus comprising:
 an optical fiber disposed in the borehole and comprising a plurality of pairs of reflectors configured to reflect light between each of the pairs of reflectors in the plurality, wherein each pair of reflectors is separated by a nominal distance L and an adjacent pair of reflectors in the plurality are separated by a nominal distance D; and 
 an optical interrogator coupled to the optical fiber and configured to sense a distance between each pair of reflectors in the plurality over time to sense the vibration, the optical interrogator comprising:
 a laser configured to emit semi-coherent light over a swept range of wavelengths to illuminate the optical fiber, wherein the semi-coherent light provides for sensing over the nominal distance L in the optical fiber between each pair of reflectors and for no sensing in the optical fiber over the nominal distance D between different pairs of reflectors; and 
 a photodetector configured to sense light from the optical fiber and provide a photodetector output signal indicative of an amount of sensed light. 
 
 
     
     
       2. The apparatus according to  claim 1 , further comprising an optical circulator having a first port coupled to the laser, a second port coupled to the optical fiber, and a third port coupled to the photodetector. 
     
     
       3. The apparatus according to  claim 2 , wherein the optical fiber comprises a plurality of optical fibers, each optical fiber in the plurality of optical fibers comprising a plurality of pairs of reflectors. 
     
     
       4. The apparatus according to  claim 3 , further comprising an optical coupler coupled to optical circulator and the plurality of optical fibers. 
     
     
       5. The apparatus according to  claim 1 , wherein the optical interrogator further comprises an optical modulator configured to modulate amplitude of light emitted by the laser. 
     
     
       6. The apparatus according to  claim 5 , wherein the optical interrogator further comprises a signal generator providing an oscillating chirped output signal to the optical modulator that modulates the light according to the oscillating chirped output signal, wherein the oscillating chirped output signal changes frequency linearly over time. 
     
     
       7. The apparatus according to  claim 6 , wherein the optical interrogator further comprises a frequency mixer coupled to the photodetector and the signal generator and configured to mix the oscillating chirped output signal and the photodetector output signal. 
     
     
       8. The apparatus according to  claim 7 , wherein the optical interrogator further comprises an analog-to-digital (A/D) converter coupled to the frequency mixer and configured to provide a digital output signal. 
     
     
       9. The apparatus according to  claim 8 , wherein the optical interrogator further comprises a digital bandpass filter coupled to the A/D converter and configured to pass a selected band of carrier frequencies to provide a filtered digital signal indicative of light interference in a selected pair of reflectors in the plurality. 
     
     
       10. The apparatus according to  claim 9 , wherein the optical interrogator further comprises a signal demodulator coupled to the digital bandpass filter and configured to detect an envelope of the filtered digital signal to provide a vibration measurement in the selected pair of reflectors in the plurality. 
     
     
       11. The apparatus according to  claim 10 , wherein the optical interrogator further comprises a plurality of sets comprising the digital bandpass filter and the signal demodulator, each set corresponding to each pair of reflectors in the plurality of pairs of reflectors. 
     
     
       12. The apparatus according to  claim 7 , wherein the optical interrogator further comprises an analog bandpass filter coupled to the frequency mixer and configured to pass a selected band of carrier frequencies to provide a filtered analog signal indicative of light interference in a selected pair of reflectors in the plurality. 
     
     
       13. The apparatus according to  claim 12 , wherein the optical interrogator further comprises (i) an analog-to-digital (A/D) converter coupled to the analog bandpass filter and configured to provide a digital output signal and (ii) a signal demodulator coupled to the A/D converter and configured to detect an envelope of the digital output signal to provide a vibration measurement in the selected pair of reflectors in the plurality. 
     
     
       14. The apparatus according to  claim 1 , wherein the laser is intrinsically configured to modulate amplitude of light emitted by the laser. 
     
     
       15. The apparatus according to  claim 1 , further comprising a controller configured to control downhole equipment in accordance with the sensed vibration. 
     
     
       16. An apparatus for sensing vibration in a borehole penetrating the earth, the apparatus comprising:
 a first pair of optical fibers and a second pair of optical fibers disposed in the borehole, each optical fiber comprising one reflector such that light is reflected between the two reflectors in the first pair of optical fibers and in the two reflectors in the second pair of optical fibers, wherein a first pair of reflectors in the first pair of optical fibers has a path distance D1 and a second pair of reflectors in the second pair of optical fibers has a distance D2 wherein D1 is less than D2; and 
 an optical interrogator coupled to the optical fiber and configured to sense a distance between each pair of reflectors in the first pair of optical fibers and the second pair of optical fibers over time to sense the vibration, the optical interrogator comprising:
 a laser configured to emit semi-coherent light over a swept range of wavelengths to illuminate the first pair of optical fibers and the second pair of optical fibers, wherein the semi-coherent light is coherent over D1 and not coherent over a differential distance D2-D1; 
 a photodetector configured to sense light from the first pair of optical fibers and the second pair of optical fibers and provide a photodetector output signal indicative of an amount of sensed light. 
 
 
     
     
       17. A method for sensing vibration in a borehole penetrating the earth, the method comprising:
 disposing an optical fiber in the borehole, the optical fiber comprising a plurality of pairs of reflectors configured to reflect light between each of the pairs of reflectors in the plurality, wherein each pair of reflectors is separated by a nominal distance L and an adjacent pair of reflectors in the plurality are separated by a nominal distance D; and 
 sensing a distance between each pair of reflectors in the plurality over time to sense the vibration using an optical interrogator coupled to the optical fiber, the optical interrogator comprising:
 a laser configured to emit semi-coherent light over a swept range of wavelengths to illuminate the optical fiber, wherein the semi-coherent light provides for sensing over the nominal distance L in the optical fiber between each pair of reflectors and for no sensing in the optical fiber over the nominal distance D between different pairs of reflectors; and 
 a photodetector configured to sense light from the optical fiber and provide a photodetector output signal indicative of an amount of sensed light. 
 
 
     
     
       18. The method according to  claim 17 , further comprising:
 modulating amplitude of light emitted by the laser using an optical modulator; and 
 providing an oscillating chirped output signal using a signal generator to the optical modulator that modulates the light according to the oscillating chirped output signal, wherein the oscillating chirped output signal changes frequency linearly over time. 
 
     
     
       19. The method according to  claim 18 , further comprising mixing the oscillating chirped output signal and the photodetector output signal using a frequency mixer to provide an analog output signal. 
     
     
       20. The method according to  claim 19 , further comprising converting the analog output signal received from the frequency mixer to a digital output signal using an analog-to-digital (A/D) converter. 
     
     
       21. The method according to  claim 20 , further comprising filtering the digital output signal using a digital bandpass filter coupled to the A/D converter to pass a selected band of carrier frequencies to provide a filtered digital signal indicative of light interference in a selected pair of reflectors in the plurality. 
     
     
       22. The method according to  claim 21 , further comprising demodulating the filtered digital signal using a signal demodulator coupled to the digital bandpass filter to detect an envelope of the filtered digital signal to provide a vibration measurement in the selected pair of reflectors in the plurality. 
     
     
       23. The method according to  claim 17 , further comprising performing a downhole action using a controller configured to control downhole equipment based upon the sensed vibration.